The national security of the United States of America (USA), along with many other countries around the globe, is at risk of attack by nuclear and/or radioactive weapons. The USA and international community need detectors to expose these threats at the borders of the nations, airports, and sea ports. Many of the detectors currently being used are large and bulky, and not acceptable to be used as portable radiation detectors. Radiation detectors may use a gamma or neutron source in order to detect if radioactive material is present in a container, building, vehicle, etc.
Neutron interrogation techniques have specific advantages for detection of hidden, shielded, or buried threats over other detection modalities in that neutrons readily penetrate most materials, providing backscattered gammas indicative of the elemental composition of the potential threat. Such techniques have broad application to military and homeland security needs. Present neutron sources and interrogation systems are expensive and relatively bulky, thereby making widespread use of this technique impractical.
One of the concerns with explosives detection and protection is that a safe distance should be maintained. Generally, it is not desirable to approach the suspected explosive. However, to detect unknown threats remotely requires a very strong source of neutrons. Generally, neutrons cannot be focused like a laser onto a target. The further away from the unknown threat, the more neutrons need to be produced because neutrons generally spray out everywhere in an uncontrolled fashion. It is quite difficult to produce enough neutrons to interrogate objects from a distance.
The crystal driven neutron source approach has been previously demonstrated using pyroelectric crystals that generate extremely high voltages when thermal cycled. Referring to FIG. 1, a prior art schematic diagram is shown of one method of neutron interrogation. A neutron source 102 produces a neutron flux 104, with an angular neutron flux/energy distribution 106. The narrower this angular neutron flux/energy distribution 106 can be, the stronger the neutron beam impacting the unidentified threat 108 can be, thereby increasing the chances of detecting a harmful threat. Prompt and delayed gammas 112, x-rays, etc., are thrown off by the unidentified threat 108 upon contact with the neutron flux 104. These prompt and delayed gammas 112 are detected by a NaI photon detector 114 or some other type of photon detector known in the art. Each impacted gamma 116 is detected by the photon detector 114 for determining if there is a real threat, and if so, what type of threat is the unidentified threat 108. Several schemes are available for neutron-based detection, including pulsed fast neutron analysis (PFNA), thermal neutron analysis (TNA), associated particle imaging (API), etc. These schemes can identify contrabands such as explosives, drugs, radioactive material, etc., through C/N/O ratios deduced from gammas released from the target for explosives and drugs, and fission related gammas for radioactive materials.
Many current neutron-based technologies are able to penetrate metal walls, casings, soil, vehicles, and are able to propagate neutrons over distance. However, current isotropic neutron sources need significant shielding in order to operate safely, the neutron sources are generally bulky, and often require large associated equipment in order to be operated. Also, these neutron sources generally lack good directional focus, e.g., it is difficult to direct where the neutrons are being sent, thereby requiring higher neutron output to be effective. Traditionally, portable neutron sources utilizing conventional HV and Penning ion sources have a characteristic size on the order of about 30 inches and weights of up to about 60 pounds. The current neutron sources using pyroelectric or pyrofusion neutron sources do not have on/off or pulsing capability of the neutron output, and run mostly steady-state at less than about 103 D-D neutrons/second (n/s), or equivalently, less than about 105 D-T n/s. D-D represents a fusion reaction that can produce neutrons, with deuterium ions onto a deuterated target. D-T represents a fusion reaction that can produce neutrons, with deuterium ions onto a tritiated target. For more information on pyroelectric properties and effects, see Sidney B. Lang, “Pyroelectricity: From Ancient Curiosity to Modern Imaging Tool,” Physics Today, August 2005.
The availability of a notably more intense, pulseable, lower weight, reduced power demanding, smaller neutron source using pyroelectric properties would open up new threat interrogation schemes utilizing neutron and/or gamma spectroscopy.